The EM Drive is special in that it is actually public knowledge and has been verified by NASA as producing anomalous thrust when powered with electricity. Of course black projects would have been all over that kind of thing with resources much higher than the public NASA Eagleworks and hobbyist projects with their garage construction and magnetrons scavenged from microwave ovens, but the point is that the EM Drive means that there is hard proof for new physics.

The attached charts show calculations by NASA Eagleworks about the suitability of the EM Drive for space travel. The EM Drive can produce continuous thrust without rocket propellant, which is a big deal in space travel. The charts assume a nuclear-powered space ship and use very, very conservative values for generated thrust per kilowatt. There is every reason to think that the actual thrust would be far higher than that once the theory is understood and new drives can be designed on that basis.

Antigravity implies a warp drive. And in this post I'm intentionally keeping things grounded and avoiding flights of speculation.